Lobster, crab and shrimp are invertebrates belonging to the class known as “crustacea”. The skeletons of crustaceans are located on the exterior surface of their bodies and are known as exoskeletons. The exoskeleton forms a hard shell which protects the animal from predators. Although rigid, the shells of crustaceans are segmented to permit movement and are softer and more flexible than the shells of bivalve mollusks such as clams or oysters.
The shell of the crustacean is attached to the underlying muscle tissue by a continuous series of intracuticle fibres that extend across the entire animal. These intracuticle fibres extend from the surface of the muscle tissue to the outer surface of the shell via pore canals in the shell. This continuous attachment renders it difficult to remove the shell by manual cutting. Accordingly, a variety of processing means have been developed to weaken the linkages affixing the shell to the body in order to facilitate removal of the shells and retrieval of the meat underneath.
Traditional methods for removing the shells of crustacean shellfish, such as shrimp and lobster, involve cooking the animals prior to peeling. Heat denatures the linkages attaching the exoskeleton to the shellfish meat, thus facilitating removal of the shells. However, following the application of heat, additional means must be employed to separate the meat from the shell. Manual extraction of the meat is time-consuming and laborious while mechanical means often cause the meat to be minced or flaked, thus limiting its potential applications. A further difficulty with using heat to facilitate loosening of the shell is that the shellfish meat underneath inevitably becomes cooked during the process. Cooking of the shellfish meat affects both its flavour and its texture thereby preventing the meat from being used in cuisines requiring raw ingredients, such as sushi. Cooking represents a particular problem in the case of lobsters due to the differing thickness in various parts of the lobster's body. Cooking periods of sufficient length to loosen the shell of the tail of the lobster often result in overcooking of the meat in the smaller sections such as the claws and the legs. The result is reduced flavour and quality in valuable sections of the animal.
Removal of the shells represents a particular problem in the case of lobster meat which is virtually impossible to extract from its shell in the raw state. Chefs requiring raw lobster meat for certain dishes will reduce the cooking time as much as possible by blanching live lobsters to loosen the shells. However, blanching fails to effect complete removal of the shells and still results in some cooking of the surface meat. Restaurants which serve fresh lobster typically maintain the animals in a live state up until the point of cooking and then serve them cooked, but still within the shell. The ability to purchase raw, but deshelled, lobster meat would greatly increase the ease and convenience of serving a variety of fresh lobster dishes.
In the case of shrimp, a common technique for facilitating removal of the shells is to permit the shrimp to “mature” for two to three days following death. During this period, the shell softens thus rendering the shrimp easier to peel. However, this method detracts from the freshness of the shrimp. Accordingly, a need exists for a process to facilitate removal of the shells of crustacean shellfish without heating the animals, and thereby cooking the shellfish meat, or leaving them to mature, thus reducing their freshness.
In recent years, attempts have been made to loosen the shells of raw lobsters by soaking them in chemicals or in enzymes to loosen the connective tissue attaching the shells to the bodies. U.S. Pat. No. 6,235,338 describes a method of removing raw meat from the head-shell of a crustacean shellfish by immersing the animal in a solution of protease enzymes. The same patent also describes an additional method involving freezing of the animal followed by vacuum aspiration. However, these methods have not delivered consistent results and have not been widely adopted by the seafood industry.
In recent years, technology has been developed which has enabled foods to be processed using high pressure treatment. Exposure of foods to high pressure has been most commonly used to eliminate bacteria and other pathogens. High pressure treatment has been used as a preservation method for a variety of different types of foods including meats, fruits and other products. U.S. Pat. No. 6,426,103 (the '103 patent) issued on Jul. 30, 2002, describes the use of high hydrostatic pressure to eliminate pathogenic organisms from raw shellfish. Foods, including seafoods, subjected to high pressure have been shown to maintain a high quality in their texture, taste and appearance.
In addition to the elimination of pathogens, the '103 patent describes release of the meat of oysters which had been subjected to pressure at a minimum of 25,000 psi for a period of 15 minutes at ambient temperature. A similar effect was observed at higher pressures for shorter periods. Oysters are bivalve mollusks having hard shells consisting of two halves. The shell of the oyster is attached to a muscle called an adductor muscle. The '103 patent discloses that, following pressurization, the adductor muscle connective tissue of the oyster was denatured to a gel. As a result, a gap appeared between the two shell halves and the oyster meat slid out without the need for manual cutting. The '103 patent does not suggest that high pressure could be used to facilitate the removal of shells of animals other than oysters.
The physiology of bivalve mollusks is quite distinct from that of crustaceans. As can be observed in FIG. 1, the meat of bivalve mollusks is attached to the shell only at the adductor muscle. By contrast, as can be observed in FIG. 2, the meat of crustacean shellfish is attached to the shell continuously across the body of the animal. Furthermore, the attachment mechanism in crustacean shellfish is complicated by the presence of innervations of external tactile sensory hairs that pass through the shell to the underlying cuticle.
Addressing this point in greater detail, crustaceans such as lobsters and crabs are mobile and hence require muscle structure and function permitting movement. This is achieved by complete and continuous attachment of muscle to the crustacean exoskeleton. In contrast, bivalves (e.g. clams, oysters, mussels and scallops) are either sessile (mussels and oysters), or move by extending a muscular foot from the shell (clams) or by propulsion (scallops). Accordingly, whereas bivalve muscle tissue is attached to the shell only at small discrete points required to close the shell (adductor muscles) or for shell formation (mantle), the majority of the shell of a crustacean possesses muscle attachments.
In lobsters, particularly, the configuration of muscle attachment is much more complex than that in bivalves. Pores are evenly distributed over the surface of the lobster shell. These pores extend through the shell and into the muscle tissue. The resulting complex matrix of pore invaginations into the muscle tissue of the lobster is partly responsible for the difficulty in removing lobster meat from the shell in an uncooked state.
The lobster shell is composed of an epicuticle, procuticle, membranous layer, epidermis and basement membrane. The muscle tissue lies beneath the basement membrane. A highly interdigitated intermediate junction occurs between the epidermis and the muscle tissue. Within the epidermis, conical hemidesomosomes are formed as invaginations of the apical cell membrane, and intracuticular fibers pass form the conical hemidesomosomes into the cuticle. Each hemidesomosome joins a muscle attachment fiber, which extends within a pore canal through the cuticle to the outer epicuticle.
In contrast, bivalve shells consist of three layers: a thin outer periostracum, a mid prismatic layer of aragonite or calcite, and an inner calcareous (nacreous) layer. The middle and inner layers contain thin layers of columnar prisms known as myostraca. The mantle and muscles of the bivalve are attached to the myostraca by strands of connective tissue.
The present invention provides a novel method for effecting detachment of the shells of lobster, crab or shrimp to facilitate their removal. It also provides seafood products obtained using this method.